Because a hash reference is already a scalar, I'm assuming that you mean you want it into a scalar *string*. Here's one way that you can exploit a scalar reference to be used as a memory file, and write the output of Data::Dumper directly to that scalar as a string value.

use warnings;
use strict;

use Data::Dumper;

my $href = {
    a => 1,
    b => [
            2,
            3,
         ],
    c => {
            25 => 'y',
            26 => 'z',
         },
    d => 4,
};

my $memfile;

open my $mfh, '>', \$memfile or die $!;
print $mfh Dumper($href);
close $mfh;

print $memfile;
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Output:

$VAR1 = {
          'c' => {
                   '25' => 'y',
                   '26' => 'z'
                 },
          'b' => [
                   2,
                   3
                 ],
          'd' => 4,
          'a' => 1
        };
[download]